One very basic aspect of face-to-face human communication is the ability of the two parties to both talk and be heard at the same time. This aspect of face-to-face communication is quite important, insofar as it enables parties to a conversation to interrupt each other and thus build substantial efficiency into their communication. It is also important in other respects, for example, in the event that one party is beginning to speak about something which may, perhaps unknown to him, be uncomfortable to a third party to the conversation or otherwise destructive of the object of the communication.
While the ability to interrupt the speech of another seems quite natural, it is an aspect of face-to-face communication not found in many electronic telecommunications systems. Indeed, the ability to both speak and be heard at the same time presents technical complications in most telecommunication systems. For example, if we consider radio frequency carrier wave transmissions, if two parties to a conversation transmit at the same time, the signals will interfere with each other causing beat frequency oscillations, feedback and the like. Systems which solve this problem and thus allow the parties to the conversation to both speak and be heard at the same time are referred to as duplex systems. This can be achieved, for example, in the case of radio frequency communication, by a pair of transmitters operating with respective receivers at two different frequencies, one assigned to each of the parties.
In contrast, until the introduction of speaker telephones, virtually all telephones were duplex systems. Generally, the operative portion of telephone systems during this period comprised the series combination of a variable resistance carbon microphone and an electromagnetic earphone. As the caller spoke over the line, a diaphragm coupled to a carbon powder compartment in the microphone caused successive compressions to be exerted against the carbon powder in the compartment thus varying the electrical resistance of the compartment. This in turn varied the current passing through the series circuit resulting in a modulation of a signal placed across the series combination. This modulated signal, modulated at the actual frequency of the voice signal being transmitted, was then sent over the telephone system to the telephone set of the other participant to the telephone conversation.
This system, which continues in use substantially unchanged from the original instruments developed by Bell in the 1870's, as noted above, inherently has a duplex characteristic. Duplex communication is achieved because the audio frequencies involved do not cause unacceptable interference with each other and because the gain of the potential feedback loop between the carbon microphone and the earphone is far less than one.
At the advent of speaker telephones, it became necessary to introduce into the telephone instrument, an audio amplifier for receiving audio signals from the telephone central office and amplifying them to drive a loudspeaker. This immediately presented the problem of preventing feedback between a microphone adjusted for sensitivity to the voice of a person who is not speaking directly into it while making the system unresponsive to audio signals introduced into the environment by the loudspeaker. To somewhat better understand this problem, it must be kept in mind that the telephone is a two-wire system used to carry both the transmitted and received signal. If the transmitted signal is thus allowed to be amplified by the amplifier which amplifies the received signal which is also carried on the same two wires, ambient noise will be amplified and feedback oscillations are likely to ensue at normal levels of speaker amplitude.
One approach to this problem was embodied in speaker phone systems which included separate microphones and loudspeakers, both of which had some directional characteristic designed to ensure that information on loudspeaker would be loud enough for the telephone use to hear while at the same time having less audio field strength at some point where the microphone was placed. Likewise, the solution involved a microphone whose sensitivity characteristic was directed toward the mouth of the individual using the system with minimal sensitivity in the vicinity of the speaker.
Thus, design objectives involved reducing the gain of the feedback loop between the microphone and the speaker to less than one with the volume control for the system set at a level which would allow easy intelligibility of the signal.
Such an approach does not, in principle, provide a commercially acceptable level of performance, as, for example, it imposes limits on the location of the parties to the conversation. Moreover, the provision of several microphones is required in order to achieved good spatial separation between the microphone and the speaker and, as a result, the system becomes somewhat cumbersome physically. As a practical matter, it was also necessary for the user to adjust the position of the various parts of the system as well as the volume on it. For persons without technical ability, successful operation of such a system was a hit or miss proposition and, in practice, even a reasonable facsimile of the best possible performance of the system was seldom achieved, with most users settling for barely operational configurations despite various electronic systems for attempting to alleviate these problems.
Another approach to this problem and one which is probably most widespread in modern communication systems is the sacrifice of duplex operation to trouble-free speaker telephone operation. Generally, these systems incorporate an electronic switch which either turns off the speaker when the user is speaking or disables the microphone when the party at the other end of the telephone is speaking and compares signal intensities when signals are being produced at both ends of the telephone conversation.
In accordance with so-called "hybrid" technology, a duplex solution to the speaker telephone problem, without the above difficulty, has been approached. Generally, such systems operate by introducing a hybrid electronic circuit which, is meant to approximate the complex impedance of the telephone system, and to produce a cancellation signal.
This cancellation signal, when added to the signal on the telephone system (comprising both the transmitted and received signal) results in generating a third signal which includes only the received signal, which third signal is, in turn, sent to the amplifier and loudspeaker of the speaker telephone system.
While such an approach would appear to provide a perfect solution to the duplex speaker telephone problem, as a matter of fact, the approach suffers from several inadequacies. Firstly, telephone system line impedances vary greatly from system to system in different parts of the country and even from exchange to exchange within the same city. Thus, it becomes necessary for the system to be installed and the complex impedance adjusted to minimize feedthrough of the signal to be transmitted into the telephone speaker amplifier. Naturally, this represents a substantial expense insofar as it involves having a technician on site for installation of the system. The increase in cost is significant enough that, for the great majority of users, such systems are not, from an economic standpoint, a practical option.
Moreover, even after such a system is installed, experience has shown that the complex impedance of the telephone lines will vary from call to call and from time to time depending upon the lines being used by the central office switching system, environmental factors, and the like. Thus, the above on site adjusted systems, at best, represent only an approximation and, for that matter, an approximation of irregular quality depending upon the nature of the particular telephone system with which they are used.